Method and device for separation at sub-ambient temperature

ABSTRACT

In a method for separation at sub-ambient temperature, a mixture of fluid at sub-ambient temperature is sent to a system of separation columns comprising at least one separation column, a fluid enriched in a lighter component of the mixture leaves the top of one column of the system and a fluid enriched in a heavier component is withdrawn from the bottom of one column of the system, the cold source of a heat pump using the magnetocaloric effect is thermally connected to a first zone of one column of the system and the hot source of the same heat pump is thermally connected to a second zone of the same or of another column of the system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2014/052241, filed Sep. 10, 2014, which claims the benefit of FR1358666, FR1358667, and FR1358668, all of which were filed Sep. 10, 2013 and are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and to a device for separation by separating at sub-ambient, or even cryogenic, temperature. The separation may be a separation by distillation and/or by dephlegmation and/or by absorption. The equipment used for this separation will be referred to as a “column”. Thus, a column may for example be a distillation or absorption column. Reduced to its simplest expression, it may be a phase separator. Otherwise, a column may also be a device in which dephlegmation takes place.

BACKGROUND OF THE INVENTION

Magnetic refrigeration relies on the use of magnetic materials that exhibit a magnetocaloric effect. Reversible, this effect is manifested by a variation in temperature when subjected to the application of an external magnetic field. The optimum ranges within which these materials are used lie in the vicinity of their Curie temperature (Tc). This is because the greater the variations in magnetization and, therefore, the changes in magnetic entropy, the greater the changes in temperature. The magnetocaloric effect is said to be direct when the temperature of the material increases when placed in a magnetic field, and indirect when it cools when placed in a magnetic field. The remainder of the description will be given for the direct case, but it is obvious to a person skilled in the art how to reapply this to the indirect case. There are many thermodynamic cycles based on this principle.

A conventional magnetic refrigeration cycle consists i) in magnetizing the material in order to increase its temperature, ii) in cooling the material for a constant magnetic field in order to dissipate heat, iii) in demagnetizing the material in order to cool it and iv) in heating the material in a constant (generally zero) magnetic field in order to absorb heat.

A magnetic refrigeration device employs elements made of magnetocaloric material, which generate heat when magnetized and absorb heat when demagnetized. They may employ a magnetocaloric material regenerator to amplify the temperature difference between the “hot source” and the “cold source”: the magnetic refrigeration is then said to be magnetic refrigeration employing active regeneration.

It is known practice to use the magnetocaloric effect to supply cold to a method for separating air by cryogenic distillation.

U.S. Pat. No. 6,502,404 describes the use of the magnetocaloric effect to supply cold (needed to provide the refrigeration balance of the method) to a cryogenic method for separating the gas of the air, the separation energy being conventionally supplied by pressurized air allows the operation of the vaporizer-condenser of the double column (it being possible for the low-pressure column to be reduced to a simple vaporizer in the case of a nitrogen generator).

SUMMARY OF THE INVENTION

The present invention tackles the problem of the transfer of heat from one place in a device for separation by distillation and/or by dephlegmation and/or by absorption at sub-ambient temperature, which place is considered to be a cold source, to another place of said device considered to be a hot source.

It has long been known to use one and the same circuit to provide both heat to the reboiler of a distillation column and frigories to the condenser of this same column. U.S. Pat. No. 2,916,888 discloses one example for the distillation of hydrocarbons.

A heat pump is a thermodynamic device that allows a quantity of heat to be transferred from a medium considered to be the “emitter” and referred to as the “cold source” from which heat is extracted, to a medium considered to be the “receiver” and referred to as the “hot source” to which the heat is supplied, the cold source being at a colder temperature than the hot source.

The conventional cycle used in the prior art for this type of application is a thermodynamic cycle of compressing—cooling (condensing)—expanding—reheating (vaporizing) a refrigeration fluid.

FIG. 12 of the document entitled “TECHNIQUES DE L'INGENIEUR—Réfrigération magnétique [Engineering techniques—Magnetic refrigeration] 2005” shows a twofold improvement in the coefficient of performance of a refrigeration system using a magnetic cycle as compared with the conventional cycle.

An ambient temperature is the temperature of the ambient air in which the method is situated or, alternatively, a temperature of a cooling water circuit connected with the air temperature.

A sub-ambient temperature is at least 10° C. below ambient temperature.

U.S. Pat. No. 4,987,744 describes a cryogenic distillation method in which a heat pump transfers heat from one point of one column, which is at a cryogenic temperature, to another point on the column, which is likewise at a cryogenic temperature. The heat pump comprises two closed refrigerant circuits thermally connected to one another, each of the circuits comprising a compression step and a cooling step using a fluid at ambient temperature (air, water).

One subject of certain embodiments of the invention provide a method for separation at sub-ambient, or even cryogenic, temperature, in which a mixture of fluid at sub-ambient, or even cryogenic, temperature is sent into a system of separation columns comprising at least one separation column, a fluid enriched in a lighter component of the mixture leaves the top of one column of the system and a fluid enriched in a heavier component is withdrawn from the bottom of one column of the system, in which the cold source of a heat pump using the magnetocaloric effect is directly or indirectly thermally connected to a first zone of a column of the system and the hot source of the same heat pump is directly or indirectly thermally connected to a second zone of the same or of another column of the system, the minimum temperature of the first zone being lower than the maximum temperature of the second zone.

According to other optional subjects:

-   -   a gas of the first zone condenses at least partially and is         possibly sent back to the first zone;     -   a liquid of the second zone is vaporized at least partially and         is possibly sent back to the second zone;     -   at least a fluid coming from the first or second zone is placed         in direct contact with a magnetocaloric material of a heat pump         using the magnetocaloric effect;     -   the exchange of heat is performed at least in part between a         fluid coming from the first or second zone and a heat-transfer         fluid that has been in contact with a magnetocaloric material of         a heat pump using the magnetocaloric effect via an exchanger;     -   the exchange of heat is performed at least in part between a         fluid coming from a first or second zone and a heat-transfer         fluid that has been in contact with a magnetocaloric material of         a heat pump using the magnetocaloric effect through an         intermediate heat-transfer circuit.     -   the heat-transfer fluid is a liquid;     -   the heat-transfer fluid does not change phase during the method;     -   the heat-transfer fluid remains at a constant pressure during         the method;     -   the heat-transfer fluid is not compressed by a compressor;     -   the heat pump does not transfer heat to outside the separation         device;     -   the heat pump transfers heat only from the hot source to the         cold source;     -   the hot source connected to the second zone operates at the         highest temperature of the heat pump;     -   the heat pump operates entirely at cryogenic temperatures;     -   the heat pump is arranged in the same cold box as the system of         columns;     -   the mixture is air;     -   the heat pump using the magnetocaloric effect condenses a         nitrogen-enriched gas in the first zone and vaporizes an         oxygen-enriched liquid in the second zone;     -   a plurality of heat pumps is employed, heat being supplied to         several heat pumps from a first zone and/or heat coming from         several heat pumps being sent to a second zone;     -   the main components of the mixture are carbon monoxide and/or         carbon dioxide and/or hydrogen and/or methane and/or nitrogen;     -   in order to produce a liquid in the bottom of the column         containing more than 97 mol % oxygen, argon is removed from the         liquid withdrawn at the bottom of the column by separating an         argon-enriched intermediate gas from the column in a         distillation column in order to produce a flow that is more rich         in argon;     -   use is made of several heat pumps using the magnetocaloric         effect, one of which is used to condense an intermediate gas         tapped off higher up the column and another is used to vaporize         an intermediate liquid from lower down the column;     -   to produce a column bottom liquid containing less than 96.5 mol         % oxygen, in which a heat pump using the magnetocaloric effect         is used to vaporize an intermediate liquid from lower down the         column.

Another subject of the invention is a device for separation at sub-ambient, or even cryogenic, temperature, comprising a system of separation columns comprising at least one separation column to which a mixture of fluid at sub-ambient, or even cryogenic, temperature is sent, a pipe for withdrawing a fluid enriched with a lighter component of the mixture from the top of one column of the system and a pipe for withdrawing a fluid enriched in a heavier component from the bottom of one column of the system, in which the cold source of a heat pump using the magnetocaloric effect is directly or indirectly thermally connected to a first zone of a column of the system and the hot source of the same heat pump being directly or indirectly thermally connected to a second zone of the same or of another column of the system, the arrangement of the first and second zones in the column or columns being such that the minimum temperature of the first zone is lower than the maximum temperature of the second zone.

According to other optional aspects, the device comprises:

-   -   means for sending a gas from the first zone to condense;     -   means for sending the condensed gas to the first zone;     -   means for sending a liquid from the second zone to vaporize at         least partially;     -   means for sending the vaporized liquid to the second zone;     -   means for placing at least a fluid coming from the first or         second zone into direct contact with a magnetocaloric material         of a heat pump using the magnetocaloric effect;     -   the exchange of heat is at least partially carried out between a         fluid coming from the first or second zone and a heat-transfer         fluid that has been in contact with a magnetocaloric material of         a heat pump using the magnetocaloric effect through an         exchanger;     -   the exchange of heat is at least partially carried out between a         fluid coming from the first or second zone and a heat-transfer         fluid that has been in contact with a magnetocaloric material of         a heat pump using the magnetocaloric effect through an         intermediate heat-transfer circuit;     -   the mixture is air;     -   the heat pump using the magnetocaloric effect is capable of         condensing a nitrogen-enriched gas in the first zone and         vaporizes an oxygen-enriched liquid in the second zone;     -   a plurality of heat pumps, means for supplying heat to several         heat pumps from a first zone and/or means for sending heat from         several heat pumps to a second zone;     -   in order to produce a column bottom liquid containing more than         97 mol % of oxygen, a distillation column for eliminating argon         from the liquid withdrawn at the bottom of the column by         separating an argon-enriched intermediate gas from the column in         order to produce a more argon-rich flow;     -   several heat pumps using the magnetocaloric effect, one of which         is used to condense an intermediate gas tapped off higher up the         column and another is used to vaporize an intermediate liquid         from lower down the column.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 represents an embodiment of the present invention.

FIG. 2 represents an embodiment of the present invention.

FIG. 3 represents a process flow diagram in accordance with an embodiment of the present invention.

FIG. 4 represents a process flow diagram in accordance with an embodiment of the present invention.

FIG. 5 represents a process flow diagram in accordance with an embodiment of the present invention.

FIG. 6 represents a process flow diagram in accordance with an embodiment of the present invention.

FIG. 7 represents a process flow diagram in accordance with an embodiment of the present invention.

FIG. 8 represents a process flow diagram in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The invention will be described in greater detail with reference to the figures.

In FIG. 1, a mixture containing at least components A, B cooled to a sub-ambient, or even cryogenic, temperature is separated in a column 3 to form a fluid, possibly gaseous, rich in volatile components A and a fluid, possibly liquid, rich in less-volatile components B. If it is desirable to transfer heat from a first zone 1 between the second and third gas-liquid contact portions to a second zone 2 between the first and second gas-liquid contact portions, this could be done using a heat pump using the magnetocaloric effect MC, the temperature of the first zone 1 being appreciably lower than that of the second zone 2, which means to say that the exchange of heat between the two zones through a simple exchanger cannot take place either because the temperature difference is negative or because the temperature difference is too small. The first zone 1 is directly or indirectly thermally connected to the cold source of the heat pump MC using the magnetocaloric effect, and the second zone 2 is directly or indirectly thermally connected to the hot source of the same heat pump MC using the magnetocaloric effect.

In FIG. 2, a mixture containing at least components A, B cooled to a sub-ambient, or even cryogenic, temperature is separated in a column 3 to form a fluid, possibly gaseous, rich in volatile components A and a fluid, possibly liquid, rich in less-volatile components B. A mixture containing at least C, D, cooled to a sub-ambient, or even cryogenic, temperature is separated in a column 5 to form a fluid, possibly gaseous, rich in volatile components C and a fluid, possibly liquid, rich in less-volatile components D. If it is desired to transfer heat from a first zone 1 between the second and first gas-liquid contact portions of the column 3 to a second zone 2 between the second and third gas-liquid contact portions of the column 5, this can be done using a heat pump using the magnetocaloric effect MC. The first zone 1 is directly or indirectly thermally connected to the cold source of the heat pump using the magnetocaloric effect MC, and the second zone 2 is directly or indirectly thermally connected to the hot source of the same heat pump MC using the magnetocaloric effect.

In FIG. 3 a compressor 7 compresses a flow A, B (for example of air considered to be a mixture mainly of oxygen and nitrogen). The compressed flow is cooled in a cooler 9 and purified in a purification unit 11 to remove the impurities (if present). The purified flow is cooled in a heat exchanger 13 to a cryogenic temperature and is split into two flows 23, 25. One flow 23 is sent to the column 3 in gaseous form and the rest 25 is cooled or, at least partially liquefied, in an exchanger 27. The cooled or at least partially liquefied flow is sent to the column 3. The column 3 has a top end condenser 15 and a bottom end reboiler 17. The condenser 15 is considered to be the first zone 1 and the reboiler 17 is the second zone 2, the heat being transferred from the first zone 1 to the second zone 2 by means of a heat pump using the magnetocaloric effect MC1. The cooling or at least partial liquefaction of the flow 25, which in part compensates for the electrical or mechanical energy introduced by the operation of the heat pump using the magnetocaloric effect MC1, can be performed directly or indirectly by means of a heat pump using the magnetocaloric effect MC4 using a cooling fluid 51, typically ambient air or cooling water or any other refrigeration system, for example a compression/expansion thermodynamic cycle.

In FIGS. 4 and 5, heat is transferred from the top of the medium-pressure column of a double air separation column to the bottom of the low-pressure column thereof. In FIG. 4, a compressor 7 compresses a flow A, B (for example air considered to be a mixture mainly of oxygen and nitrogen). The compressed flow is cooled in a cooler 9 and purified in a purification unit 11 to remove the impurities (if present). The purified flow is split into two. A part 23 cooled in a heat exchanger 13 to a cryogenic temperature is sent to the bottom of the medium-pressure column 3. The rest 123 has its pressure boosted in a pressure booster 41, is partially cooled in the heat exchanger 13, and is then expanded in an inlet turbine 43, driving the pressure booster 41. The expanded air is sent to the low-pressure column 5, directly or indirectly thermally connected to the medium-pressure column 3 through a heat pump using the magnetocaloric effect MC. Other means of producing cold other than the intake turbine may be considered.

A liquid flow 61 enriched in B (oxygen) and a liquid flow 63 enriched in A (nitrogen) are withdrawn from the medium-pressure column 3 and sent to the low-pressure column 5.

In FIG. 4, the column 3 has no top end condenser in the column and the column 5 has no bottom reboiler in the column. Gaseous nitrogen 47 is withdrawn from the column 3 and split into two. One part 49 is heated up in the exchanger 13. The rest 51 is sent to the heat pump using the magnetocaloric effect MC where it condenses (possibly partially) and the condensed nitrogen is sent to the top of the column 3. Liquid oxygen from the bottom of the column 5 is also sent to the heat pump using the magnetocaloric effect MC where it vaporizes (possibly partially) before being sent back to the column 5. Thus, the heat pump using the magnetocaloric effect MC replaces both the top condenser and the bottom reboiler, making it possible in particular to reduce the overall height of the system of columns 3, 5. Gaseous oxygen 53 is withdrawn from the column 5 by way of product, is heated up in the exchanger 13 and compressed in the compressor 55. It is obvious to a person skilled in the art that liquid oxygen may be withdrawn from the column either by way of liquid product or so that it can be pumped and then vaporized in the exchange line 13 against the pressure-boosted air.

Nitrogen 57 is withdrawn from the top of the low-pressure column 5, heated up in the subcooler 45 and in the exchanger 13 before being used at least in part for the regeneration of the unit 11.

FIG. 5 shows a more conventional alternative form in which the column 3 has a top end condenser 15 and the column 5 has a bottom end reboiler 17. The transfer of heat between the two takes place indirectly by means of a heat pump using the magnetocaloric effect MC, a heat-transfer fluid, for example a liquid, coming from the heat pump using the magnetocaloric effect MC being supplied to the condenser, a heat-transfer fluid, for example a liquid, coming from the heat pump using the magnetocaloric effect MC being supplied to the vaporizer, it being possible for the two heat-transfer fluids to be the same.

Compared with the conventional double column configuration, the use of a heat pump employing the magnetocaloric effect as illustrated in FIGS. 3, 4 and 5 to perform the distillation makes it possible to save approximately 20% on energy, notably by reducing the pressure of the flow A, B in the compressor 7 needed for separation.

FIG. 6 shows a device similar to that of FIG. 3, except that it comprises an argon-producing column 103 supplied from the column 3. This argon column 103 may actually produce an argon-enriched fluid or alternatively may output the argon-enriched fluid in a residual flow. The condenser of the argon column 103 is supplied with a liquid coming from the column 3, withdrawn on the “distillation equivalent plates” around the introduction of liquid air, above the introduction of air 23. Said liquid is vaporized (at least partially) in the condenser of the argon column 103 in order to refrigerate the argon column 103, and is then reintroduced into the column 3 underneath the air supply 23. The argon column 103 is connected to the column 3 underneath the reintroduction of said vaporized liquid. In that case, the oxygen 29 withdrawn from the bottom of the column 3 may have a purity of more than 97 mol %.

FIG. 7 shows a device similar to that of FIG. 6, in which the heat pumps using the magnetocaloric effect MC2, MC3 replace the argon production column 103 and the top condenser thereof. Some of the heat exchanged at the condenser 15 of the column 3 is transferred directly or indirectly to the cold source of the heat pump using the magnetocaloric effect MC3. The heat pump using the magnetocaloric effect MC3 has by way of hot source a liquid withdrawn between the liquid air inlet and the gaseous air inlet 23, which is at least partially vaporized and reintroduced into the column 3 under the gaseous air supply 23. The heat pump using the magnetocaloric effect MC2 has as its cold source a gas withdrawn below the level of said reintroduction, which is at least partially condensed and reintroduced at the level of said reintroduction. Some of the heat exchanged at the vaporizer 17 of the column 3 comes directly or indirectly from the hot source of the heat pump using the magnetocaloric effect MC2. The remaining heat exchanged at the condenser 15 of the column 3 is transferred directly or indirectly to the cold source of the heat pump using the magnetocaloric effect MC1. The remainder of the heat exchanged at the vaporizer 17 of the column 3 comes directly or indirectly from the hot source of the heat pump using the magnetocaloric effect MC1. The heat pumps using the magnetocaloric effect MC1, MC2 and/or MC3 may be fully or partially combined into one and the same device.

The method in FIG. 7 has an energy performance identical to FIG. 6 in the case where the argon-enriched fluid is discharged in a residual flow. It allows an energy saving of around 7% in comparison with FIG. 3. In that case, the oxygen 29 withdrawn at the bottom of the column 3 may have a purity of more than 97 mol %.

FIG. 8 depicts a device similar to that of FIG. 3 but comprising an intermediate reboiler. In that case, heat is transferred indirectly from the condenser 15 to both the two reboilers 17, 71 through two heat pumps using the magnetocaloric effect MC1, MC2. Part of the heat exchanged at the condenser 15 of the column 3 is transferred directly or indirectly to the cold source of the heat pump using the magnetocaloric effect MC2. The heat exchanged at the intermediate reboiler 71 situated below the gaseous air inlet 23 comes directly or indirectly from the hot source of the heat pump using the magnetocaloric effect MC2. The remainder of the heat exchanged at the condenser 15 of the column 3 is transferred directly or indirectly to the cold source of the heat pump using the magnetocaloric effect MC1. The heat exchanged at the vaporizer 17 of the column 3 comes directly or indirectly from the heat pump using the magnetocaloric effect MC1. In that case, the oxygen 29 withdrawn at the bottom of the column 3 may have a purity of less than 96.5 mol %. The heat pumps using the magnetocaloric effect MC1 and MC2 may be fully or partially combined within one and the same device.

By comparison with the conventional double column configuration with double vaporizer in the low-pressure column, the use of heat pumps using the magnetocaloric effect as illustrated in FIG. 8 to perform the distillation allows an energy saving of up to about 20%.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. A method for separation at sub-ambient temperature, the method comprising the steps of: sending a mixture of fluid at sub-ambient temperature into a system of separation columns comprising at least one separation column; and withdrawing a fluid enriched in a lighter component of the mixture from the top of one column of the system and withdrawing a fluid enriched in a heavier component from the bottom of one column of the system; wherein the cold source of a heat pump using the magnetocaloric effect is directly or indirectly thermally connected to a first zone of a column of the system and the hot source of the same heat pump is directly or indirectly thermally connected to a second zone of the same or of another column of the system, wherein the minimum temperature of the first zone is lower than the maximum temperature of the second zone.
 17. The method as claimed in claim 16, in which a gas of the first zone condenses at least partially and is possibly sent back to the first zone.
 18. The method as claimed in claim 16, in which a liquid of the second zone is vaporized at least partially and is possibly sent back to the second zone.
 19. The method as claimed in claim 16, in which at least a fluid coming from the first or second zone is placed in direct contact with a magnetocaloric material of a heat pump using the magnetocaloric effect.
 20. The method as claimed in claim 16, in which the exchange of heat is performed at least in part between a fluid coming from the first or second zone and a heat-transfer fluid that has been in contact with a magnetocaloric material of a heat pump using the magnetocaloric effect via an exchanger.
 21. The method as claimed in claim 16, in which the exchange of heat is performed at least in part between a fluid coming from the first or second zone and a heat-transfer fluid that has been in contact with a magnetocaloric material of a heat pump using the magnetocaloric effect through an intermediate heat-transfer circuit.
 22. The method as claimed in claim 16, in which the mixture is air.
 23. The method as claimed in claim 22, in which the heat pump using the magnetocaloric effect condenses a nitrogen-enriched gas in the first zone and vaporizes an oxygen-enriched liquid in the second zone.
 24. The method as claimed in claim 16, in which a plurality of heat pumps is employed, heat being supplied to several heat pumps from a first zone and/or heat coming from several heat pumps being sent to a second zone.
 25. The method as claimed in claim 16, in which the main components of the mixture are carbon monoxide and/or carbon dioxide and/or hydrogen and/or methane and/or nitrogen.
 26. The method as claimed in claim 16, in which in order to produce a liquid in the bottom of the column containing more than 97 mol % oxygen, argon is removed from the liquid withdrawn at the bottom of the column by separating an argon-enriched intermediate gas from the column in a distillation column in order to produce a flow that is more rich in argon.
 27. The method as claimed in claim 16, in which use is made of several heat pumps using the magnetocaloric effect, one of which is used to condense an intermediate gas tapped off higher up the column and another is used to vaporize an intermediate liquid from lower down the column.
 28. The method as claimed in claim 16, to produce a column bottom liquid containing less than 96.5 mol % oxygen, in which a heat pump using the magnetocaloric effect is used to vaporize an intermediate liquid from lower down the column.
 29. The method as claimed in claim 16, in which the hot source operates at the highest temperature of the heat pump.
 30. A device for separation at sub-ambient temperature, comprising a system of separation columns comprising at least one separation column to which a mixture of fluid at sub-ambient temperature is sent, a pipe for withdrawing a fluid enriched with a lighter component of the mixture from the top of one column of the system and a pipe for withdrawing a fluid enriched in a heavier component from the bottom of one column of the system, in which the cold source of a heat pump using the magnetocaloric effect is directly or indirectly thermally connected to a first zone of a column of the system and the hot source of the same heat pump being directly or indirectly thermally connected to a second zone of the same or of another column of the system, the arrangement of the first and second zones in the column or columns being such that the minimum temperature of the first zone is lower than the maximum temperature of the second zone. 